People are interested in poly lactic acid (PLA) since the raw material thereof, lactic acid is derived from biological source and can be biodegraded, and PLA has the similar process ability and mechanical properties as common plastics such as PP and PE. PLA has a broad range of application and enormous market potential, however, compared with the aromatic polyesters such as PET, PLA possesses higher brittleness and poor impact resistance, which has limited its wider applications.
The properties of PLA can be improved by modification, for example, by melt-polycondensation with aromatic polyester, or by melt coupling, thereby the resulting copolymer possessing the properties of both of the two polymers. However, the melt-polycondensation should be carried out at high temperatures and under high vacuum. The reaction condition is tough, the transesterification reaction is inevitable and the distribution of each block would be random in the copolymer chains. Meanwhile, the racemization of PLA tends to occur during the melt-polycondensation, and the PLA block exists usually in amorphous form. As a result, PLA possesses poor crystallizability, even no crystallizability. In addition, PLA may be degraded under the high temperatures and high vacuum reaction condition, and the content of PLA in the product is much lower than the feedstock (Licheng Tan et al, J Therm Anal Calorim, 2010, 99:269-275; Journal of Applied Polymer Science, 2008, 108:2171-2179).
The multi-block copolymers between PLA and PET which was disclosed by Chinese patent (CN101338025A) were synthesized by coupling hydroxyl terminated PET and PLA prepolymer using diisocyanate as the coupling agent. The low melting temperature PET was firstly synthesized by introducing the long chain aliphatic diol. Then, the coupling reaction was carried out in the molten state at 140-210° C. However, since this reaction was performed by coupling method in the molten state, the product is the mixture of polymers with complex structures and is difficult to separate, it's impossible to obtain block copolymer with the definite structure.
CN101134807A has disclosed the following method: the block copolymers of PLA is synthesized via ring-opening polymerization of L-lactide under 110-180° C. in molten state with the low-melting-point hydroxyl terminated prepolymer as initiator. Because of the use of the low Tm macroinitiator, this molten state polymerization reaction could be carried out under relatively lower temperature. However, all of the melting temperature of aromatic polyester such as PET, PTT, PBT, is higher than 220° C., with the Tm of PET being about 260° C. Thus, if lactide ring-opening polymerization is initiated by polyester of high melting temperature such as PET, PTT, PBT under molten state, the transesterification reaction would happen, and the degradation and racemization of lactide or PLA are inevitable. Therefore, it is not feasible to synthesize the block copolymers of PLA and polyesters with higher melting temperature by melt-polycondensation.